1. Field of the Invention
The present invention concerns a bidirectional transmission system, especially one using optical fiber, employing a single carrier for both transmission directions.
The present invention cad be used among other things to provide bidirectional transmission on a link, especially an optical fiber link, between two equipments of the link (called terminals) when one of these terminal equipments (called a user terminal) must be as simple as possible and in particular must not incorporate any carrier source, especially any optical carrier source.
A user terminal of this kind may be a subscriber terminal of a telecommunication network such as the telephone network, for example, especially in the context of connecting subscribers to the network using optical fiber. This application will be taken by way of specific example in what follows.
2. Description of the Related Art
Various techniques have been considered for providing bidirectional transmission via optical fiber between subscriber terminals and their local telephone network central office.
For each of the techniques mentioned below, only one connection need be considered (between a subscriber and the central office), even if a set of subscribers can be connected to the central office by the same link, in particular by multiplexing.
FIG. 1 summarizes a first technique.
In this technique the central office CL and the subscriber terminal TA are connected by two optical fibers and information is transmitted from the central office to the subscriber (this is called the downward direction) on a fiber f.sub.1 separate from the fiber f.sub.2 on which information is transmitted from the subscriber to the central office (this is called the upward direction).
The drawbacks of this first technique are the duplication of all the transmission equipments (two optical senders S'.sub.1, S'.sub.2 each comprising an optical source, two optical fibers f.sub.1, f.sub.2 and two optical receivers D'.sub.1, D'.sub.2) and the presence of an optical source at the subscriber terminal.
A second technique summarized in FIG. 2 and described in the document IEEE Global Telecommunications Conference, H. Kobrinski, L. S. Smoot, and T. J. Robe, "A passive photonic loop architecture employing wavelength-division multiplexing" uses the great bandwidth of optical fiber to provide bidirectional transmission on a single fiber f between the central office CL and the subscriber terminal TA. In addition to the optical senders S'.sub.1, S'.sub.2 and the optical receivers D'.sub.1, D'.sub.2, optical couplers C'.sub.1, C'.sub.2 are then required to distinguish between the two transmission directions, respectively that to the central office and that to the subscriber terminal. Two wavelengths may be assigned to a call, a wavelength .lambda..sub.d for the downward direction and a wavelength .lambda..sub.m for the upward direction. Because of the nature of light, the same wavelength may be used for both transmission directions, however, as described in the document Electronic Letters, Vol. 20, No. 18, pp. 722-723, 1984, A. P. McDonna, D. J. McCartney, and D. B. Mortimore, "1.3 .mu.m bidirectional optical transmission over 31 km of single-mode fibre using optical couplers". The major disadvantage of this type of technique is again the presence of an optical source in the subscriber terminal.
A so-called "ping-pong" variant of the second technique reserves some timeslots for transmission in the upward direction and other timeslots for transmission in the downward direction. Various drawbacks are then incurred over and above the one mentioned above:
simultaneous transmission in both directions is not possible; PA1 synchronization of transmission times allowing for the propagation times between the central office and the subscriber is required. PA1 The wavelength assigned to transmission in the upward direction must be controlled to minimize interference with other calls, whether or not it is different from that used for transmission in the downward direction. PA1 The use of specific wavelengths for transmission in the upward direction requires each subscriber to have a different send equipment (including an optical source) compatible with the receive equipment at the central office.
As previously mentioned, a system which uses an optical carrier generated at the user terminal is unattractive, especially for a subscriber connection system, for two reasons:
It is difficult to guarantee wavelength stability because of the distance between subscribers and between each subscriber and the central office.
A technique for dispensing with an optical source at the subscriber terminal described in the document Electronic Letters, Vol. 23, No. 18, pp. 943-944, 1987, H. Kobrinski and S. S. Cheng, "Laser power sharing in the subscriber loop" is summarized in FIG. 3.
As in the previous techniques, data d'.sub.1 from the central office is transmitted to the subscriber by modulating an optical carrier of wavelength .lambda..sub.d from an optical source in a sender S'.sub.1. For transmission in the upward direction another optical source S".sub.1 in the central office sends to the subscriber on the same fiber f' a carrier at a wavelength .lambda..sub.m different than .lambda..sub.d. Using a modulator M'.sub.2 the subscriber modulates this optical carrier with the data d'.sub.2 to be sent. The modulated optical carrier, which is still at the wavelength .lambda..sub.m although this wavelength is now denoted .lambda..sup.r.sub.m (r=relayed) is relayed, in this example over the same fiber, to the central office.
In a system based on this technique the absence of any optical source at the subscriber terminal eliminates the need for wavelength control between all network subscribers. Control of the light sources at the central office is still required, however, but this is easier to implement because the sources are all located in the central office. However, a system of this kind has the drawback of requiring two optical sources rather than one source at the central office.
A variant of this technique using a single optical carrier is described in the document Electronics Letters, Vol. 22, No. 10, pp. 528-529, 1986, T. H. Wood, E. C. Carr, B. L. Kasper, R. A. Linke, C. A. Burus and K. L. Walker, "Bidirectional fibre-optical transmission using a multiple-quantum-well (MQW) modulator/detector". This variant uses a common component for modulation and detection at the subscriber terminal. This component cannot function simultaneously as a modulator and as a detector, however, so that transmission in the upward direction cannot take place at the same time as transmission in the downward direction.
A technique enabling simultaneous bidirectional transmission of optical signals between a subscriber and a central office using a single optical carrier described in the document Conference on Optical Fiber Communication (Atlanta, Ga., U.S.A., 24-26 February 1986), Technical Digest, paper MH4, pp. 14-15, P. J. Duthie, M. J. Wale, J. Hankey, M. J. Goodwin, W. J. Stewart, I. Bennion and A. C. Carter, "Simultaneous bidirectional fiber-optic transmission using a single source" is summarized in FIG. 4.
A single optical source included in a sender S'.sub.1 at the central office is used for simultaneous bidirectional transmission between the central office and a subscriber terminal. For transmission in the downward direction the carrier from the optical source is amplitude modulated, the resulting light signal is transmitted over a fiber f' and some of the light, sampled by a coupler C'.sub.2, is detected at the subscriber terminal by the receiver D'.sub.2. For transmission in the upward direction the subscriber uses a directional coupler type modulator M'.sub.2 to superimpose his signal d'.sub.2 on the non-detected part of the light used for transmission in the downward direction from the coupler C'.sub.2. The resulting optical carrier is reflected by a mirror for transmission to the central office at the same wavelength .lambda.' now denoted .lambda.'.sup.r (r=reflected) via the same fiber f'. To enable the central office to reconstitute the signal d'.sub.2 sent by the subscriber, the transmission bit rate in the upward direction must be lower than that in the downward direction, however, so this technique cannot be used to transmit data at the same bit rate in both directions. Another drawback of this technique is that, in the case of data transmitted in digital code, the information conveyed in the downward direction must be coded to avoid long sequences of zero values during which the subscriber could not send (because he would not be receiving any light).